This invention relates to a process for producing a pigment-coated phosphor in which the color filter particles (hereinafter referred to as "pigment") adhere fastly to the surfaces of the phosphor particles and which can be used for the production of the high-contrast cathode-ray tubes for color television receivers.
The prior art for producing the pigment-coated phosphors includes the S. A. Lipp process (U.S. Pat. No. 3,886,394) and the G. S. Lozier et al process (U.S. Pat. No. 4,049,845). According to the former process, a gelatin-adsorbed phosphor is contacted with a polyvinylpyrrolidone-adsorbed pigment so that the pigment adheres to the phosphor surface. The thus produced pigment-coated phosphor, however, is unsatisfactory in pigment-phosphor adhesion and it was found that the pigment is liable to separate from the phosphor during the steps of preparation, ageing and coating of the phosphor slurry practiced in a usual way.
According to the Lozier et al process, a phosphor having adsorbed therein colloidal particles of a latex is mixed with a pigment or a pigment having adsorbed therein colloidal particles of a latex in an aqueous medium and the mixture is coagulated to obtain a pigment-coated phosphor. Various means are disclosed for effecting said coagulation, such as changing the pH of the medium, changing the medium temperature, adding polyvalent cations to the medium, adding a water-soluble organic solvent, etc.
Some of the present inventors have previously proposed a process for producing a pigment-coated phosphor by mixing a phosphor, a pigment and an anionic emulsion in an aqueous medium, adding thereto a cationic emulsion and, if need be, making the mixture system neutral or weakly acidic (Japanese Patent Appln. No. 151988/76 corresponding to U.S. patent appln. Ser. No. 862156).
Both the Lozier et al process and the process of the above-said Japanese application are capable of producing pigment-coated phosphors wherein the pigment adheres fastly to the phosphor, but it was found that the pigment-coated phosphors obtained according to these processes still have some serious problems. For instance, when the obtained dry cakes are passed through, for example, a 300-mesh sieve (Tyler standard), they are sparingly passable through such a sieve, resulting in a very low workability. Also, dispersibility of the phosphor coating slurry is poor and no good filling is provided when the fluorescent film is formed, so that no homogenous coating film is obtained, resulting in reduced luminous brightness in the same film thickness, and there also takes place color mixing or cross-contamination when the phosphor is applied on the cathode-ray tube for color television receivers. (The term "Cross-contamination" means a phenomenon that takes place when a second or third luminous component phosphor to be applied on the face plate of the cathode-ray tube is coated with the slurry, exposed and developed to form the dots or stripes thereof, and such phosphor remains on the already formed dots or stripes of other luminous component phosphor.)
Some improvements for sieving workability have been proposed, such as a method for making the mixture system non-adhesive by hardening a resin binder included therein, but it was found that when said resin binder is hardened, it becomes brittle to cause separation of the pigment from the phosphor during ageing of the phosphor slurry.